Process and apparatus for controlling a counter-current electromigration installation



M. CHEMLA 3,257,293

INTER-CURRENT June 21, 1966 PROCESS AND APPARATUS FOR CONTROLLING A 00 ELEGTROMIGRATION INSTALLATION 2 Sheets-Sheet 1 Filed April 10. 1962 June 21, 1966 M. CHEMLA 3,257,293

PROCESS AND APPARATUS FOR CONTROLLING A COUNTER-CURRENT ELECTROMIGRATION INSTALLATION 2 Sheets-Sheet 2 Filed April 10. 1962 United States Patent 3,257,293 PROCESS AND APPARATUS FOR CONTROLLING A COUNTER-CURRENT ELECTROMIGRATION INSTALLATION Marius Chemla, Maisons Alfort, Seine, France, assignor to Commissariat a lEnergie Atomique, Paris, France Filed Apr. 10, 1962, Ser. No. 186,538 Claims priority, application France, Apr. 3, 1959, 791,228; May 9, 1961, 861,294, Patent 1,230,639 Claims. (Cl. 204--1) This application is a continuation-in-part of my application Ser. No, 14,090, filed Mar. 10, 1960, now abandoned.

The present invention relates to a process and apparatus for the control of a counter-current electromigration installation comprising a bath of fused salts, of the type wherein the metal which tends to be deposited or which is deposited at the cathode, is redissolved, in the ionic state, in the electrolytic bath by the action of an added material appropriate to the nature of the bath and that of the metal. Y

The process is characterised in that the electrolysis current is controlled by the dissolving of the metal at the cathode by means of a system consisting of a reference electrode, a measuring apparatus sensitive to the difference in potential in the voltaic chain cathode/electrolyte/reference electrode, and appropriate means actuated according to the readings of this apparatus and acting in such a manner as to start, vary, or interrupt the electrolysis current, in order to ensure the precise neutralisation of the cathode compartment.

This process enables the disadvantages of a possible deposition of said metal on the cathode, such as those resulting from the corrosive action of said deposition, to be overcome.

This process is particularly suitable for an installation for isotopic enrichment working by electromigration of the isotopic cations in a counter-current of electrolyte, the counter-current being produced by the said addition.

The process according to this invention, as well as the apparatus for carrying out. this process, are based on the polarisation of the cathode when the metal is deposited thereon or when the composition of the bath in its vicinity varies, the reference electrode being, naturally, situated in a zone where it will remain insensitive to the disturbances experienced by the cathode; this polarisation is revealed by the difference in potential V which then appears between the cathode and the reference electrode arranged in the cathode compartment, this instantaneous actual difference in potential V corresponding to the electromotive force of the voltaic chain formed by the cathode plus any deposition of metal thereon, the fused salt and the reference electrode.

In order to simplify the idea, it will be assumed that the cathode and the reference electrode are of the same nature, for example both of graphite, so'that the said voltaic chain is reduced to the chain graphite/fused salt/ graphite, the electromotive force of which is zero when there is no metal deposited on the cathode or when the composition of the bath has not varied.

On the other hand, if metal is deposited at the cathode or if the composition of the bath has varied in the vicinity of the cathode, an electromotive force appears between the cathode and the reference electrode, of the order of one to two volts, and may be superimposed on the difference' in potential caused bythe lines of force of the electric current causing the electromigration; in order to measure this electromotive force with precision it is preferable to stop the electrolysis current for a few moments. When the electrolysis current is stopped, a measuring apparatus connected between the cathode and the refer- Patented June 21, I966 ence electrode indicates the instantaneous actual difference in potential V between these two electrodes and consequently provides information concerning the possible presence of a deposit of metal on the cathode or of a disturbance in the vicinity of the cathode; this information is transmitted to means including a system of relays which act in such a manner as to halt the electrolysis it the cathode compartment is not exactly neutralised. Thus the continuous addition of the added material ensuring the neutralisation of the cathode compartment is controlled by the electrolysis current; in this manner, the irregularities in or the accidental cessation of this supply have no harmful influence, such as corrosion of the apparatus, on the course of the countercurrent electromigration.

According to one embodiment of the invention, provision is made to start the electrolysis when there is still no deposit at the cathode, for a pre-determined period. At the end of this period, the electrolysis current is stopped and the potential of the cathode is measured; if it is zero or low, that is to say if, at that moment, there is no metal deposited" on the cathode, the electrolysis is started again for the same period; if it is not zero, the electrolysis remains interrupted until all the metal deposited is dissolved, then it is started again for a fresh period equal to the previous one.

In this embodiment, the process of the invention is characterized by the following modes of operation taken separately or in combination:

(1) The electrolysis having been stopped, one of the relays is actuated when the difference in potential in the voltaic chain is zero or low, and it then causes the restarting of the electrolysis;

(2) this relay is' of the delay type and the electrolysis is thus interrupted at the end of a predetermined period;

(3) the electrolysis thus remains interrupted so long as the difference'in potential in the voltaic chain indicates a polarisation of the cathode:

(4) One of the relays enables the electrolysis to be stopped definitely, after the provisional interruption thereof, if the time necessary for the dissolving of any metal deposited is too long;

(5) One of the relays permits 'the electrolysis to be started again with an electrolysis current of lower strength in such a manner as to slow down the rate of the deposition of the metal.

According to another embodiment of the invention the device is notably simplified by measuring the regulating potential difference between the cathode and the anode being the reference electrode.

They also comprise several other complementary arrangements to be used preferably simultaneously, notably an arrangement consisting in protecting the contact galvanometer or other-device, during the electrolysis, by means of a resistance adapted to be 'short-circuited automatically by a relay upon completion of the electrolysis, and another arrangement consisting in applying the invention to any system (including systems other than electromigration system) wherein the occurrence or the variation of an asymmetry between electrodes is used for controlling regulation apparatus.

This invention is concerned more particularly with certain forms of application (notably the one in which the invention is applied to countercurrent type electromigration systems) and also with certain forms of embodiment of the aforesaid arrangements; still more particularly, this ponent elements necessary for their practical actuation.

In order that the invention may be more fully understood various examples of the operation of the process and of apparatus for controlling an installation for the separation of the isotopes of lithium by counter-current electromigration in fused lithium will now be described, by way of illustration, only with reference to the accompanying diagrammatic drawings in which.

FIGURE 1 is a diagrammatic representation of an embodiment of the invention and shows the position of the reference in the cathode compartment of the cell;

FIGURE 2 shows a detail of one of the relays of the control apparatus illustrated in FIGURE 1;

FIGURE 3 is a wiring diagram showing an electromigration system utilizing a bath of molten salt, for example for separating lithium isotopes, the system being constructed according to the teaching of this invention; and

FIGURE 4 is a fragmentary wiring diagram illustrating one portion of the system according to a modified embodiment.

Only the elements necessary for an understanding of the invention are illustrated in the figures, the corresponding elements in the various figures having the same references.

FIGURE 1 is a diagrammatic illustration of an installation for the isotopic enrichment of lithium by electromigration in a bath of used lithium bromide with the addition of potassium bromide. The electrolytic cell comprises a cathode compartment 1 and an anode compartment 2 connected to the cathode compartment by a bridge 3 containing a refractory porous material forming a diaphragm 4. The bromine is liberated at 5 and the supply of bromine to the cathode compartment takes place at 6. The cell is supplied with a current of suit-able strength and voltage. For this purpose, the current originating from the mains 7 is fed into an auto-transformer 8 then into a rectifier 9. The circuit also includes an ammeter 10 and a voltmeter 11. It is thus possible to supply the electrolytic cell directly with direct current. The electrolytic cell is arranged in a furnace, not illustrated, in order that the salts to be electrolysed can be melted and maintained in the fused condition.

In FIG. 1 the cathode 12 is cylindrical in shape and along its axis are arranged the bromine supply tube and a reference electrode 13 which, in principle, is identical in nature to the cathode 12, so that, in the absence of a deposit at the cathode, the difference in potential due to the chain cathode/fused salt/reference electrode is zero. If the two electrodes are of a different nature, allowance must be made for the fact that when the cathode is depolarised, there is a certain difference in potential E between them, called normal. Connected between the cathode 12 and the reference electrode 13 is a measuring apparatus 14 sensitive to the polarisation of the cathode 12.

The cathode 12, the reference electrode 13 and the anode 15 are of graphite. The electrolytic bath consists of a mixture containing 70% by weight of LiBr and 30% of KBr; the mixture of the salts is maintained molten at 460 C. The diaphragm 4, which is 10 cm. long, consists of a filling of Zirconia in the form of grains 1 mm. in diameter. The electrolysis current is 0.4 A.; the bromine is liberated in the anode compartment 2; the supply of bro-mine to the cathode compartment 1 effects the establishment of the counter-current and the redissolving, in the ionic state, of the lithium in the electrolytic bath. In the absence of the electrolysis current, the difference in potential between the cathode 12 and the reference electrode 13 acts on a contact-type galva nometer 14; this is mounted in series with a variable resistance 16a and is shunted by means of a resistance 16b. The movable part of said galvanometer 14 displaces a needle 17 in front of a scale 18 provided with a contact 19, the position of said contact on the scale 18 corresponding to a difference in potential substantially equal to zero, for example less than 0.2 v., or more generally to the difference in potential E existing, in the absence of the electrolysis current, between the cathode 12 and the reference electrode 13 when there is no metal deposited on the former and when said electrodes are of different natures (that is the normal potential difference). Let it be assumed that the electrolysis current is interrupted and that the dissolving of the metal deposited on the cathode is completed; the needle 17 then encounters the contact 19 and a delay relay 20 is instantaneously actuated; the latter starts the generator of electrolysis current for a period of forty-five minutes, representing the duration of the delay in the interruption of the relay and at the end of this time, the electrolysis current is interrupted by the relay 2%.

The relay 20 comprises a liquid switch 21 and a solenoid 22 with a vertical axis, the core 23 of which is movable; the displacement of this movable-core is connected to the rocking movement of the switch 21 by means of an appropriate mechanical connection 24.

FIGURE 2 shows the switch 21 consisting of an ampoule separated into two compartments 25 and 26 by means of a partition 27 in which there is provided an aperture 28. Partition 27 is mounted for limited oscillating moment on pivot 29 as seen in FIGURE 2.

Switches of this type are known and permit the slow passage over a given period of time of a certain quantity of mercury from the compartment 25 to the compartment 26 when the ampoule is in the position illustrated in FIGURE 2; when the ampoule rocks about the horizontal axis 2?, all the mercury then contained in the compartment 26 returns almost instantaneously when partition 27 rotates to open compartment 26 into compartment 25. Contact is maintained between flexible conductors 30 and 31 so long as any mercury remains in the compartment 25.

The relay 2!] starts the generator of electrolysis current in the following manner: when the needle 17 encounters the contact 19, the compartment 26 which was in its low position and contained all the mercury is raised by the upward displacement of the core 23 due to the passage of the current through the closed circuit IMNOPJ and the mercury abruptly flows into the compartment 25 which is then in the low position, thus permitting the passage of the electrolysis current through the closed circuit IMABRQJ. Since current is passing through the electrolytic cell, the galvanometer measures the resulting difference in potential V between the cathode and the reference electrode, which is therefore greater than the value E in such a manner that the needle 17 leaves the contact 19; current no longer enters the solenoid 22 and the switch 21 rocks again. The compartment 25 which contains all the mercury then comes into the high position, these two rocking movements follow one another very quickly and the mercury then passes slowly into the compartment 26 which is in the low position; the duration of the passage of the mercury is a function of the internal characteristics of the switch 21 and in this example a period of forty-five minutes has been selected. This time has lapsed when the last drops of mercury leave the compartment 25, the circuit IMABRQJ is then opened at Q, that is at the switch 21, and the latter is back in its initial position; the electrolysis current is no longer passing and the galvanometer 14 again measures the difference in potential due to the voltaic chain reference electrode/electrolyte/ cathode. If, at this moment, this potential is not greater than B, the delay relay 20 is again actuated and the previous cycle recommences.

If, after the interruption of the electrolysis current, there is a deposit of metal at the cathode, this difference in potential is greater than E. Since the addition of the substance producing the counter-current continues, the dc posited metal dissolved progressively and as soon as the potential of the cathode is no longer greater than B, the contact galvanometer 14 again actuates the delay relay 20 and the cycle begins again.

In certain cases, the time required to dissolve the cathodic deposit is such that the potential of the cathode does not become zero again until the end of a long period, for example of the order of four or five hours, which indicates the existence of a fault in operation. A delay 32 then intervenes and finally disconnects the whole electrolysis installation; this system renders it possible, in particular, to obviate defective operation in the bromine supply to the cathode, such as the accidental stopping of this supply.

The delay relay 32 is identical to the delay relay 20; it comprises a switch 33, a solenoid 34, a core 35 and mechanical connecting means 36; the contact N between flexible conductors 37 and 38 is effected every time the needle 17 touches the contact member 19 in such a manner as not to disturb the operation of the relay 20.

When current is passing through the electrolytic cell, since the shunt circuit RTSU is supplied, the compartment 39 to which the conductors 37 and 38 lead is in its low position and contains the mercury, so that the switch 33 is closed.

The delay relay 32 acts in the following manner: when the relay 20 interrupts the electrolysis current, the solenoid 34 is no longer energised and the switch 33 rocks, the compartment 39 being raised to the high position; the mercury then takes about forty-five minutes to pass into the compartment 40.

If the relay 20 is actuated less than forty-five minutes after the interruption of the electrolysis current, the latter again passes through the cell and the solenoid 34 is energised in such 'a manner that the switch 33 rocks again before all the mercury has passed intothe compartment 40; in this case, the switch 33 is permanently closed and the delay relay 32 does not affect the operation of the delay relay 20.

If the relay 20 has not been actuated by the galvanometer 14 during the forty-five minutes following the interruption of the electrolysis current, all the mercury in the compartment 39 passes into the compartment 40' and the switch 33 opens; the circuit of the solenoid 22 is then interrupted and when the needle 17 touches the contact member 19, the current cannot be restored; the switch 21 remains open and the whole installation is stopped and since the switches 21 and 33 are in a state of equilibrium, the installation is finally switched off. Intervention by the operator enables the installation to be re-started after checking the supply of additional substance.

The control device described above has proved completely effective in protecting the apparatus despite several interruptions in the supply of bromine and it has enabled the electromigration apparatus to be operated for more than one hundred days without the slightest incident.

Example 1.The device described above with reference to FIGURE 1 was used. The whole of the electrolytic cell was permanently placed in an electric furnace which maintained the electrolyte bath at a temperature in the region of 460 0., whether the electrolysis current was passing or had been interrupted. This external heating was reinforced by the Joule effect in the molten mass when the electrolysis current was passing.

Example Il.The control device of FIGURE 1 is likewise used, but the delay relay 20 had an additional function of stopping or re-starting the auxiliary reheating system at the same time as it stopped or restarted the electrolysis current. Actually, since the electrolytic cell was of large dimensions, the heat losses were low and it was not necessary, when electromigration was taking place, to provide heating beyond that due to the passage of the current through the molten mass; this was no longer so when the electrolysis current was interrupted for any length of time, since the mass could then have solidified and caused deterioration of the electrolytic cell and it is for this reason that provision was made for starting reheating at this moment under the action of the delay relay 20 and for the interruption of this reheating, likewise under the action of the delay relay 20, when the electrolysis current was again passing through the cell.

The dimensions of the cell are sometimes large enough for the heating of the mass by the Joule effect to become excessive; a cooling system is then provided which is started, under the action of the delay relay 20', at the same time as the electromigration; the cooling is interrupted, likewise under the action of the delay relay 20, at the same time as the electromigration.

.The operation of the auxiliary cooling and heating system may likewise be made subject to the temperature of the molten mass by means of suitable measuring apparatus, such as pyrometers.

Numerical data relating to Examples I and II are given below:

The current density selected is a function of the structure of the electrodes and is in the range between 0.5 and 1 a./cm.

The coating results in a voltage measuring by the g-al vanometer 14 of about 3 v.; for security sake the electri cal contact of the galvanometer is set so that the electrolysis current is cut off as soon as this voltage is in excess of 0.5 v.;

The weight of the molten salt bath is in the range of 50 g., the cathodic compartment having 20* g. from this weight;

The rate of flow of bromine during the electrolysis process is about 10 g./h.

Example III.--A modification of the above-described process and apparatus enables the neutralis-ation of the cathode compartment to be controlled without interrupting the electrolysis current. The reference electrode is placed in a Zone such that its potential is not disturbed too much by the lines of force of the electrolysis current. In this case, the potential difference of the voltaic chain may be measuring during the electrolysis and act, by means of a sensitive measuring apparatus, on relays which cause the electrolysis current to vary continuously in such a manner as to maintain said electromotive force a little below the value corresponding to the beginning of deposition of the metal.

Example I V.-Another modification of the process and apparatus, similar to the modification of Example III, also renders it possible not to stop the electrolysis, but differs as followsz The electrolysis current varies in a discontinuous manner, the periods of smaller electrolysis current causing the dissolving of any metal deposited at the cathode, while the changeover to a larger current takes place as soon as the instantaneous actual potential difference in the said voltaic chain resumes its normal value E.

The apparatus of Examples I to IV can be used whatever the cation metal subjected to electromigration (for example, lithium, potassium, uranium) and whatever the anion (for example nitrate, bromide, chloride; according to the circumstances, the addition at the cathode may be the halogen in the case of a halogen anion, or a salt such as ammonium nitrate, in the case of a nitrate anion).

According to a last embodiment of this invention, and more particularly to those embodiments as well as to those forms of embodiment of its various component elements which seem to constitute the preferred arrangements for example of the type incorporating a countercurrent type electromigration system of the type illustrated by the FIGURES 1 to 4 and aiming at regulating the electrolysis automatically as a function of the deposit of metal on the cathode, the following or a similar procedure is adhered to.

Safety means adapted to detect the presence of metal deposit on the cathode, that is, the cathodic polarization resulting therefrom, have already been disclosed by the FIGURE 1 these means being adapted to start or on the contrary to stop the electrolysis depending on whether In other words, the means as specified in the first embodiment subject the electrolysis current to the neutralization of the metal at the cathode.

It was found according to this invention that the system could be simplified considerably by taking this potential difference simply between the cathode and the anode, instead of between the cathode and a reference electrode other than the anode.

Moreover, in addition to the resulting simplification, a greater safety of operation is achieved since with the present invention any troubles likely to result from the proximity of the cathode and the reference electrode are definitely avoided. Finally, the cathode space may be reduced accordingly.

The principle on which this arrangement of this invention is based is that when two electrodes of same type, for example graphite electrodes, are immersed in a bath having a homogeneous composition, in the absence of electrolysis current, their potential difference is zero, and that when metal is deposited on the cathode their potential difference is equal to the electromotive force of the voltaic chain: cathode-i-metal/molten salt/ anode.

Therefore, according to this last embodiment of the invention use is made essentially of a measuring apparatus or galvanometer responsive to the above mentioned potential difference and disposed between the anode and cathode, the measurement being effected preferably after switching off the electrolysis current. Thus, the apparatus operates jointly suitable relay means arranged for example in the manner set forth this embodiment, the assembly being capable of stopping the electrolysis when the deposit formed on the cathode becomes abnormally important and requires a momentary stop of the electrolysis process to enable the bromine (in the example contemplated) to. redissolve the metal. When this dissolution is completedas observed from the reduction to zero or to a predetermined value of the potential difference to be measured--the electrolysis is resumed under the control of said relay means.

Of course, according to a preferred arrangement and with due consideration for the location of the measuring apparatus between the anode and the cathode, this apparatus must be protected against the electrolysis current for example by inserting a resistance in the circuit of this apparatus under the control of an additional special relay responsive to said current.

The various relay means may be connected for example according to provide the cycle of operations which has been already described. It will be noted that another cycle of operation be adapted.

In the accompanying drawing the wiring diagram of FIG. 3 incorporates the same time-lag relays 20 and 32 as already described about FIG. 1, with the same refer ence numerals, the windings being shown at 22, 34.

Relay 20 is a control relay and its winding 22 is connected in series with the contacts 17, 19 of the measuring apparatus or galvanometer 1 4, these two contacts engaging each other when the'po-tential difference is either zero or attains a predetermined value E, this condition corresponding to the front position of the switch 21 of this relay. 1

This switch 21 controls the electrolysis process and is common to three circuits:

The circuit of winding 8 of the transformer controlling through the rectifier 9 the supply of electrolysis current; the circuit incorporating the winding 34 of the second relay 32; and

The circuit incorporating the winding 41 of a third relay controlling the protection of the measuring apparatus, its back and front contacts R and T operating in conjunction with a variable protective resistance 42, another resistance 16 having the same function as in the aforesaid embodiment.

The contacts or switches 21, 33 of relays 20, 32 are of the so-called time-lag type and consist for example of two compartments (as shown at 25, 26 in the case of switch 2 1) between which a mass of mercurycan circulate either instantaneously in one direction or with a certain time-lag in the opposite direction, as already explained in the aforesaid embodiment.

The two switches 21 and 33 are shown in their inoperative position in which they open contacts 30, 31 or 37, 38 but only after a certain time-lag corresponding to the delayed flow of mercury therein.

In brief, the three windings 8, 34 and 41 are mounted in parallel across the terminals 7 through said switches 21 controlling the electrolysis. The switch 33 is inserted in the circuit control-ling the first relay 20 so that when this switch opens the system is stopped.

This assembly operates as follows:

Initially, the cathode being fully depolarized, in the absence of electrolysis current, the galvanometer 14 is not deflected, its pointer 17 engaging the end contact 19, thus causing the energization of relay 20 which becomes operative and closes the switch 21 instantaneously. This relay 20 controls simultaneously the supply of electrolysis current and the energization of relays 32 and 41. Under these conditions, the contact galvanometer 14 is connected to the electrodes through the medium of resistance 42, but the electrolysis voltage causes its deflection to a degree sufficient to. move the pointer 17 away from contact 19, thus restoring relay 20 to its inoperative condition. However, since this relay 20 is of the delayed breaking type even in its de-energized condition it keeps supplying electrolysis current, on the one hand, and energizing current to relays 32 and 41, on the other hand, during a predetermined time period. Upon completion of this time period the switch 211 of relay 20 opens the circuit, thus stopping the electrolysis and restoring the relays 32 and 41 to their back or inoperative position.

As relay 41 is de-energized, the galvanometer 14 will measure directly the potential difference between the anode and cathode. If this difference is zero or below a predetermined value E, the pointer 17 engages the end contact 19, thus re-energizing relay 20, and the abovedescribed cycle is resumed.

If the potential difference between anode and cathode is not zero or if it exceeds the aforesaid limit value E, thus revealing the presence of metal on the cathode, the galvanometer pointer remains deflected and the electrolysis is discontinued until the cathode polarization ceases, the potential difference between anode and cathode becoming again zero or lower than the aforesaid value E; the pointer 17 re-engages the fixed contact 19 and another cycle is resumed as before.

' However, if the neutralization time of the deposited metal is too long, thus showing that an incident or other trouble took place beforehand in the bromine supply circuit, in this last case the relay 32 becomes operative for definitely breaking the electrolysis current supply circuit which cannot be restored again.

In fact, relay 32 is of the delayed breaking type like relay 20 and is responsive to the electrolysis current as explained hereinabove; as the electrolysis current is broken this relay is thus restored to its back position, but its switch 33 is kept closed during a predetermined time period, for example fifteen minutes.

Upon completion of this time period the switch opens the general electric supply of the safety device and henceforth no relay can be energized unless an operator resets the device.

FIG. 4 illustrates a modified embodiment of the circuit connections of the contact galvanometer 14. In this alternate embodiment the galvanometer .14 is constantly connected to the cathode through the protective resistance.

and galvanometer 14 protected against the voltage across During the electrolysis relay 41 is energized the electrodes by the resistance 42. However, as in the preceding circuitry, upon completion of each cycle the relay 4-1 is triggered and shortacircuits the resistance 42 whereby the galvanometer 14 is connected directly across the electrodes.

By thus short-circuiting the resistance 42 it is possible to provide a relay 4-11 of relatively small dimensions even in the case of high-power cells utilizing a very strong electrolysis current.

This relay 411 should of course be selected among those having a rather short response time in order to ensure an immediate protection of the galvanometer when the electrolysis contact is established.

Finally, it will be seen that the improvements characterizing this last embodiment preserve the principle of the invention, that is, the automatic control of the electrolysis as a function of the cathode polarization, but with asimpler and safer mounting.

However, it should be emphasized that inaddition this invention contemplates the general application of the above defined means to any systems wherein it is desired to detect the occurrence of an asymmetry between electrodes.

Although it is apparent that in the case contemplated this asymmetry was due to the deposit of .metal on the cathode, other asymmetries may occur without any metal deposit. Thus, in certain molten salts no metal deposit will take place but an asymmetry may result from a variation in the composition of the bath in the vicinity of one of the electrodes and would be revealed by the formation of a concentration pile.

Thus, for instance, in the case of nitrates, no metal is deposited but a side-reaction takes place which is attended by the formation of an alkaline bath. Similarly, in the case of certain other salts, the metal deposited is soluble in the bath of molten salt.

' In all cases the device described hereinabove will detect the occurrence of an asymmetry between the anode and cathode compartments, a feature rather difiicult to achieve with the devicedescribed and represented by FIGURE 1.

Moreover, the phenomena contemplated may also take place in an aqueous solution.

Finally, it will be readily understood by those skilled in the art that the safety or control means according to this invention are also efiective 'on any regulation or servo-control means adapted to meet, prove or restore the conditions of operation.

Therefore, irrespective of the form of embodiment contemplated in practice, and more, particularly in the specific application contemplated herein, an electromigration system of the type set forth represented by the FIG- URE l, which operates in a manner readily understood from the foregoing so that any further description of its mode of operation is deemed unnecessary, may be established which will be characterized by many advantageous features in comparison with existing and known systems of the type concerned, the features including inter alia: I

Atfording a greater simplification of the system, due to the suppression of the auxiliary reference electrodes;

ensuring a greater safety of operation by avoiding the inconveniences resulting from troubles likely to derive from the use of an auxiliary reference electrode;

and, finally, affording smaller over-all dimensions.

Obviously, and as clearly apparent from the foregoing, this invention should not be construed as being limited to its forms of application or its forms of embodiment, whether as a whole or in the case of specific component elements thereof, which are shown and described herein, as many modifications and alterations may be brought thereto without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. Apparatus for controlling a counter-current electromigration in a bath of fused salt, an anode and a cathode in the bath, means for continuously adding a substance at said cathode capable of dissolving in the ionic state the metal deposited at said cathode, circuit means for supplying an electrolysis current between said anode and cathode, means for melting the salt to be electrolysed, a reference electrode adjacent said cathode, potential difference measuring means connected to said cathode and to said reference electrode, switch means in said circuit means opened by said measuring means, a first relay including a first delay switch connected between said switch means and said electrolysis current supply means whereby said electrolysis current is periodically interrupted by said delay switch and reinstated by said switch means and a second relay including a second delay switch connected across said circuit means, said second delay switch being connected to said first relay whereby the electrolysis current is terminated when the deposited metal fails to dissolve within a predetermined time.

2. Apparatus as described in claim 1, said measuring means comprising a galvanometer connected between said cathode and said reference electrode, said switch means comprising an electric contact for said galvanometer closed by said galvanometer only when the potential difference has a value corresponding to the absence of a deposit of metal at the cathode.

3. Apparatus as described in claim 1, wherein said first delay switch includes a solenoid for returning said switch to closed condition.

4. Apparatus as described in claim 3, further including a heating system for the bath adapted to be started and stopped by said first delay switch.

5. Apparatus as described in claim 1 further including a protective electric resistance connected between said cathode and said measuring means and a safety relay connected across said circuit means to short-circuit automatically said resistance when the electrolytic process ceases.

6. Apparatus as described in claim 5 further including a third relay adapted to stop the electrolysis process when the continuation of a potential dilference between the anode and the cathode upon cessation of the electrolysis current exceeds a predetermined time period.

7. Apparatus as described in claim 6 further including a feed transformer in said circuit means and a switch actuated by said control relay incorporated both in the circuit including said feed transformer and the circuits 8. Apparatus as described in claim -5 in which said protective resistance associated with said measuring means is adapted to be switched off by said safety relay during the measurements.

9. Apparatus as described in claim 6 in which said switches controlling said first and second delay switches are of the time-lag type.

10. A process for controlling counter-current electromigration between an anode and a cathode in a bath of fused salt, comprising continuously adding a substance at the cathode to redissolve the metal deposited at the cathode, temporarily interrupting, after a predetermined period of time, the electrolysis current between the anode and the cathode, measuring the potential deviation of the cathode caused-by the deposited metal, reinstating the electrolysis current when said measured potential dilference declines with the continued addition of the said substance to a predetermined level corresponding to the absence of deposited metal at the cathode and cyclicly carrying out the steps of interrupting the electrolysis current, measuring the potential difference and reinstating the electrolysis current.

11. The process of claim 10 wherein the electrolysis current is terminated after a preset period of time if means for supplying an electrolysis current between said anode and cathode, means for melting the salt to be electrolysed, said anode forming a reference electrode, potential difference measuring means connected to said cathode and to said anode reference electrode, switch means in said circuit means opened by said measuring means, a first relay including a first delay switch connected between said switch means and said circuit means whereby said electrolysis current is periodically interrupted by said first delay switch and reinstated by said switch means and a second relay including a second delay switch connected across said circuit means, said second delay switch being connected to said first relay whereby the electrolysis current is terminated when the deposited metal fails to dissolve within a predetermined time.

13. Apparatus as described in claim 12, said measuring means comprising a galvanometer connected between said cathode and said anode reference electrode, said switch means comprising an electric contact for said galvanometer closed by said galvanometer only when the electrolysis current is interrupted and when the potential difference has a value corresponding to the absence of a deposit of metal at the cathode.

14. Apparatus as described in claim 12, said first delay switch comprising a delay opened switch and a solenoid for returning said switch to closed condition.

15. Apparatus as described in claim 14 including a heating system for the bath adapted to be started and stopped by said first delay switch.

References Cited by the Examiner UNITED STATES PATENTS 2,566,308 9/ 195 1, Brewer 204l 2,759,8-87 8/1956 Miles 204-496 2,873,237 2/ 1959 Lamberton et a1 204-1 2,918,420 12/1959 Sabins 204--l96 3,025,224 3/ 19-62 Kepes 2041 OTHER REFERENCES Verdie-k, et al.: Journal of Physical Chemistry, vol. 46 (1942), pp. 344352.

JOHN H. MACK, Primary Examiner.

JOSEPH REBOLD, WINSTON A. DOUGLAS,

Examiners.

B. JOHNSON, T. H. TUNG, Assistant Examiners. 

10. A PROCESS FOR CONTROLLING COUNTER-CURRENT ELECTROMIGRATION BETWEEN AN ANODE AND A CATHODE IN A BATH OF FUSED SALT, COMPRISING CONTINUOUSLY ADDING A SUBSTANCE AT THE CATHODE TO REDISSOLVE THE METAL DEPOSITED AT THE CATHODE, TEMPORARILY INTERRUPTING, AFTER A PREDETERMINED PERIOD OF TIME, THE ELECTROLYSIS CURRENT BETWEEN THE ANODE AND THE CATHODE, MEASURING THE POTENTIAL DEVIATION OF THE CATHODE CAUSED BY THE DEPOSITED METAL, REINSTATING THE ELECTROLYSIS CURRENT WHEN SAID MEASURED POTENTIAL DIFFERENCE DECLINES WITH THE CONTINUED ADDITION OF THE SAID SUBSTANCE TO A PREDETERMINED LEVEL CORRESPONDING TO THE ABSENCE OF DEPOSITED METAL AT THE CATHODE AND CYCLICLY CARRYING OUT THE STEPS OF INTERRUPTING THE ELECTROLYSIS CURRENT, MEASURING THE POTENTIAL DIFFERENCE AND REINSTATING THE ELECTROLYSIS CURRENT. 